1. Field of the Invention
This invention relates to a system and method for tracking inventory and freight using the global positioning satellite system.
2. Description of the Related Art
The present invention utilizes the global positioning satellite system (GPS) to determine the location of freight, inventory, packages or the like (“freight”) in a holding area, such as a freight terminal, railyard, airport, warehouse or other storage area. Knowledge of GPS and freight or inventory problems and procedures is useful for an appreciation of the present invention. U.S. patent application Ser. No. 07/804,368 entitled “Golf Distance Measuring System and Method” (incorporated by reference) describes inter alia a system for tracking golf carts and players on a golf course using GPS and is analogous to the present invention which tracks freight.
THE GLOBAL POSITIONING SATELLITE SYSTEM
GPS is a spaced based system of satellites which can provide an infinite number of receivers accurate three dimensional position (i.e. horizontal location and altitude), velocity, and time. A general understanding of GPS is useful to appreciate the operation of the present invention. Numerous books and articles are available on GPS operation and theory. See e.g., GPS—A Guide to the Next Utility, Trimble Navigation (incorporated by reference for background).
The GPS system is an umbrella of satellites circling the earth passively transmitting signals. Each satellite has a very accurate atomic clock which is periodically updated. A GPS receiver with an accurate clock can identify a satellite and determine the transit time of the signal from the satellite to the receiver. Knowing the transit time and knowing that the speed of light is 186,000 miles per second enables a calculation of the distance from the satellite to the receiver. The signal carries with it data which discloses satellite position and time of transmission, and synchronizes the GPS receiver with the satellite clocks.
As a GPS receiver locates 3 or 4 satellites it determines its distance from each satellite. The intersection of these 3 or 4 spheres enables a precise location of the receiver (and some compensation for timing errors in the receiver's internal clock). The GPS system should have 21 satellites and 3 spares once the system is fully deployed. The full constellation of 24 ellites was declared operational in 1994.
There are basically two types of GPS receivers—P (precision) code and C/A (coarse availability) code. P code is for government use only and requires specialized equipment. C/A code receivers are becoming widely available with the continuing deployment of GPS satellites. One difficulty with C/A code receivers is that the government from time to time intentionally degrades the satellite signals—so called “selective availability.” With selective availability turned on horizontal accuracy is on the order of 50-100 meters. With selective availability disabled horizontal accuracy can improve to around 15 meters, often better than 5 meters.
There are several methods presently available for improving the horizontal accuracy of GPS. One method is called “differential” and generally involves sending a correction signal from a base station located at a known coordinate. For example, the U.S. Coast Guard has placed a number of GPS base stations at known locations around the U.S. coast region. These base stations compare their GPS computed positions with the known coordinates of their location to calculate a differential correction. This differential correction is then broadcast to any GPS receiver in range. This correction may be a position correction, but normally the correction is to the timing signal for each individual satellite so that GPS receivers looking at different satellites may calculate their own correction. This is a “wide area” approach. A “local area” approach is also often used for differential correction where a private gps base station is positioned at a known location and broadcasts a private or local correction.
Another correction approach which has not yet matured but is promising is a so-called “pseudolite” correction. With a pseudolite a GPS transmitter transmits a timing signal much like a GPS satellite. See, The Use of Pseudo-Satellites For Improving GPS Performance, D. Klein, B. Parkinson, Navigation (1984), reprinted Vol. III GPS Navigation p. 135 (1986); Optimal locations of Pseudolites for Differential GPS, B. Parkinson, K. Fitzgibbon, 30 Navigation J. No.4, winter 1986-87 (incorporated by reference for background). The pseudolite transmits from a known location on or near the standard GPS carrier frequency (e.g. L1 or L2) to appear to the GPS receiver like another gps satellite. The difference is the pseudolite does not have normal gps errors (or at least minimal), such as ephemeris, ionospheric, multipath, etc., and more importantly, the pseudolite does not have the intentional degradation, selective availability. Additionally, a differential correction signal can be added to the pseudolite signal if desired. A primary benefit of use of pseudolites is that unlike normal differential correction, pseudolites do not require a separate communications channel. That is, the pseudolites appear as another satellite channel to the receiver. Another benefit is that the timing data from the pseudolite channel is known to be much more precise.
FREIGHT TRACKING SYSTEMS
Consider a railyard, airport, or sea terminal. A number of railcars or freight containers are constantly on the move into and out of the terminal. The cargo is generally of high value and often transit time is time critical. Indeed, transit time can be very costly when considering a large number of freight containers delayed by even a day extra. The incidence of misdirected or misplaced freight or cargo can add significantly to the shipping costs. Keeping track of where a particular freight container is located is a daunting task considering the often dynamic nature of a freight terminal and repositioning of the cargo.
Tracking inventory in an industrial yard is a similar problem. In manufacturing, it is desirable to track the location and availability of finished goods. Most systems use some form of manual label tracking or bar codes to track the inventory. Unfortunately, manual tracking often requires a person to traverse the inventory and scan labels to identify the presence of the inventory.